ABSTRACT ? Overall Thousands of patients benefit each year from the lifesaving impacts of organ transplantation, but this benefit is hampered by the requirement of preventing rejection of the donor graft. Establishing immunologic tolerance in organ transplant recipients using life-long immunosuppressive therapy, in particular, the lymphocyte depletion immunosuppression strategy has been a critical component of all successful approaches. Following lymphocyte depletion, immune cells proliferate to fill the newly created gap in the immune repertoire, a process referred to as homeostatic repopulation. During this process the immune microenvironment critically influences the ensuing immunologic repertoire and can detour the response to allogeneic donor tissue either toward rejection or tolerance. We propose that the post-depletional fate of lymphocytes depends on its differentiation state, its exposure to antigen, and the effect of exogenous factors driving or inhibiting its growth. Within this context, we advance a generalizable framework for repopulation: recollective homeostasis, positing that the lymphocyte repertoire at any given time is determined by an ongoing process of memory acquisition and deletion. Importantly, depletion accelerates turnover, creating an intense period of opportunity during which we propose this process can be manipulated, exploiting relative differences in cell susceptibility to create, over time, a state of sustained allospecific hypo-responsiveness. This proposal seeks to capitalize on this window of opportunity to reshape the immune repertoire towards tolerance through the work of two interrelated projects investigating specific repopulation events supported by an administrative core and a scientific core for immune profiling. Project 1 will focus on depletional T lymphocyte repopulation, while Project 2 will focus on B lymphocyte/plasma cell/antibody modulation. These projects are scientifically distinct but are interactive in methodologies and mechanisms. Specifically, both projects will employ our major histocompatibility complex (MHC)-defined, nonhuman primate (NHP) renal transplantation model that provides rigorous translational evaluation of potential novel tolerance induction strategies. Homeostatic repopulation is a central theme in our work that serves to generalize our approach beyond tolerance protocols, to broader immunosuppressant- and viral-induced lymphopenic states. This proposal will also improve upon our understanding of the role of costimulation blockade and the drug belatacept in tolerance induction; their ability to enhance the specificity and tolerability of anti-rejection therapy and to downregulate alloantibody has been a longstanding focus of our research. Our proposed studies will build on two decades of collaborative experience investigating both depletion and costimulation blockade in NHPs. The knowledge gained from these studies will further our understanding of tolerance mechanisms in NHPs and humans, extend our understanding of T and B cell biology, and deepen our understanding of transplant immunosuppression.